Wireless communication systems provide for dynamic exchange of data, controls and information. Such wireless communications however are subject to interference in many forms. The sum of the interferers in the information environment is sometimes referred to as the electromagnetic environment (EME). For example, military operations are generally executed in an information environment that is increasingly complicated by the EME that can include passive and active interference including jamming and electronic countermeasures.
Oscillators, which comprise passive components (e.g. a resonant cavity) and associated circuitry, are building blocks of many of the systems that operate in radio-frequency (RF) and microwave systems (e.g. radar transmitters and receivers, wireless communications systems, Analog-to-Digital Converters (ADCs), etc.). The performance of these systems is often dependent on the performance of oscillators used therein.
For instance, the performance of many microwave systems is limited by the master oscillator phase noise, which is also known as “jitter” when represented in the time domain. For example, in RF communications equipment, information is typically modulated on a carrier frequency, which results in side-bands, as illustrated in FIG. 1. Since the carrier frequency spectral width is finite, this limits channel capacity. By reducing phase noise, spectral content can be narrowed, resulting in higher channel capacity. Additionally, phase noise (manifesting as time jitter) in analog to digital conversion equipment results in uncertainties in both sampling time and the sampled signal value, as illustrated in FIG. 2A.
As a further example, in radar applications, the reflected signal from a moving target is often a frequency modulated and normally severely attenuated version of the transmitted radar signal, due to the Doppler effect, atmospheric attenuation, and low target radar cross-section. Therefore, the detection probability of low radar cross-section targets is fundamentally limited by the reference oscillator phase noise. In military skirmishes, where targets may move at speeds higher than 300 mph, the reflected signal is normally at an offset larger than 10 kHz away from the transmitted signal. This normally translates to a phase noise requirement of <140 dBc@10 kH offset at x-band for high fidelity target detection. However, as threats have evolved over the past few decades, radar has been widely applied for surveillance purposes, and it has become increasingly important for radar to detect slower-moving targets with lower radar cross-section, such as vehicles, and individuals. When dealing with such targets, the reflected signal is generally only hundreds of hertz away from the transmitted signal, as shown in FIG. 2B. This imposes much more stringent phase noise requirements on reference oscillators.
Unfortunately, oscillators that are able to provide state of the art phase noise at microwave/mm-wave frequencies that would tend to mitigate such issues also tend to be relatively large in size and weight and require a substantial amount of power to function (e.g. cryogenically cooled bench-top instruments). As a practical matter, the size, weight, power consumption, and, in some cases, ancillary requirements (e.g. cryogenic cooling systems) of such devices tends to preclude their use in defense and other applications having similar packaging and power limitations (e.g. on aircraft). In comparison to such state of the art oscillators, low Size Weight and Power (SWaP) oscillators, such as that shown in FIG. 3, rely on ovenized quartz oscillators and frequency conversion techniques to generate the required microwave frequency output. They offer phase noise performance that is on the order of 100× worse than state of the art benchtop solutions. This severely limits the performance of the RF and microwave systems of which such devices are a component, ultimately limiting their effectiveness.
What is needed, therefore, are improved methods of integrating low loss passive components with associated circuitry that could be applied to oscillators and allow for state of the art phase noise performance at microwave/mm-wave frequencies in a low SWaP package.